Low-swirl Flame Research Articles

Three-dimensional flame chemiluminescence tomography reconstruction based on outer contour pre-reconstruction

The computed tomography of chemiluminescence (CTC) can be used to reconstruct a three-dimensional (3D) flame chemiluminescence field to obtain information about the spatial characteristics of the flame. However, additional information is needed to solve the ill-posed inverse problem of the CTC due to the constraints such as economy of CTC system and the number of views. In this study, a PR-SART algorithm is proposed for 3D flame reconstruction by combining the flame outer contour pre-reconstruction model with the simultaneous algebraic reconstruction technique (SART). The influence of the number of pre-reconstruction iterations is analyzed in numerical studies. The reconstruction performance of the SART algorithm is compared with the PR-SART algorithm for two flame structures under various numbers of views and noise conditions. Finally, an OH* chemiluminescence imaging system consisting of 8 ultraviolet (UV) cameras is developed, and evaluated through use of reconstructing the 3D structure of low-swirl flames. Numerical and experimental studies indicate that the proposed algorithm and CTC system are effectively capable of removing the reconstruction error in the flame-free region, improving the reconstruction quality, and reducing the computational cost.

Flame synthesis of nanoparticles based on high flux electrostatic atomization burner.

This study presents an innovative electrostatic spray flame synthesis (ESFS) reactor that combines the advantages of electrostatic spray and flame synthesis for precise spray control and efficient single-step continuous synthesis. To overcome the limitations of conventional ESFS systems, which often suffer from low atomized precursor flux, we successfully demonstrated a high-flux disk electrostatic atomizer coupled low-swirl flame reactor, achieving a precursor flux of up to 30 ml/h about 30 times higher than that of typical ESFS devices. The atomized precursor being rapidly carried away from the burner is undergoing high-temperature pyrolysis and particle formation through lifted premixed turbulent flames. The ESFS system provides extensive control over parameters such as flame temperature, equivalence ratio, residence time, initial droplet sizes, and precursor concentrations. For illustrative purposes, the ESFS system was utilized to synthesize silica nanoparticles, demonstrating the capability of synthesizing nanoparticles with various properties. By manipulating the collection position and height, the particle size has made a substantial leap from the nanoscale to the micrometer level. This remarkable achievement underscores the system's enormous potential for precise particle size regulation and one-step synthesis of complex structured thin films.

Investigation of differential diffusion in lean, premixed, hydrogen-enriched swirl flames

Hydrogen combustion is a promising alternative to fossil fuel combustion in an effort to reduce our carbon footprint. However, hydrogen combustion is prone to thermodiffusive instabilities largely dependent on differential diffusion, a phenomenon that can lead to higher probabilities of flashback in industrial burners, given hydrogen’s high reactivity and diffusivity. This paper evaluates low-swirl flames of methane and air enriched with hydrogen to highlight the onset of differential diffusion. Testing was conducted in a fully controllable swirl burner, where bulk velocity Uav = 13 m/s and swirl number S = 0.6 were kept constant for each hydrogen–methane blend to isolate increases in flame surface area from increases in turbulence intensity. Furthermore, each fuel blend of hydrogen and methane is evaluated at the same laminar flame speed of SLo = 0.267 m/s to isolate flame stretch effects on the turbulent burning rate. Combined hydroxyl (OH) PLIF and stereoscopic PIV at the National Research Council of Canada were used to analyze the OH fluorescence in a 2D-3C velocity field for each flame condition. High-speed PIV at McGill University was used to resolve local flame phenomena, such as local flame displacement velocity and flame stretch rate. Using these techniques, it can be observed that the flame displaces axially in response to turbulent flame speed while exhibiting increases in flamefront wrinkling. This increased corrugation due to flame stretch is highlighted in the PDFs of local curvature and κSf and is further evidenced by a shift towards positive curvatures (κ> 0) for increasing H2 volume fraction. This trend suggests that there is a strong correlation with increases in turbulent burning rate and positive curvature as a result of differential diffusion, but it is not necessarily a control mechanism of the most forward propagating points proposed by the theory of leading points.

Large Eddy Simulation and Acoustic Analysis of Combustion Instability in an Annular Combustor with Low-Swirl Flames.

Self-excited combustion instability in an annular combustor with low-swirl flames is studied with a combination of large eddy simulation (LES) and acoustic solvers. Acoustic analysis with a Helmholtz solver provides an estimate of frequencies and modal structures in the annular combustor. LES gives detailed modal dynamics for specific instability modes. Combustion instabilities in the annular combustor including longitudinal, spinning, and standing modes are successfully captured in a single LES. Numerical results show that the instability modes are not constant; they switch among these modes randomly and rapidly. The flow oscillates back and forth in phase with the largest pressure amplitude located near the outlet of the injectors for the longitudinal mode. The azimuthal instability oscillates in the 1A2L mode of the annular system. In the spinning mode, the pressure antinodes move forward while the modal structure keeps constant. For the standing mode, the locations of pressure antinodes are fixed in the annular combustor and the fluctuations at the pressure antinodes keep out of phase. The near-zero value of the mean spin ratio indicates that the dominant azimuthal mode is the standing mode. The azimuthal modes captured by LES are in good agreement with that predicted by Helmholtz solver in terms of frequency and modal structure. The maximum deviation of the predicted frequency is less than 5%. This adds values before the low-swirl injector is placed into the actual annular combustor.

Investigation of the three-dimensional structure of low-swirl methane-hydrogen flames with multiple UV cameras-based OH* chemiluminescence tomography

Blending hydrogen with natural gas is useful for reducing CO2 emissions, but this alters the flame structure, creating challenges for flame stability. Numerous researches on methane-hydrogen flame structure have been carried out based on two-dimensional combustion diagnostic techniques such as flame chemiluminescence and planar laser-induced fluorescence. In this paper, a low-cost OH* chemiluminescence imaging system consisting of 8 ultraviolet (UV) cameras is proposed to reconstruct the three-dimensional (3D) structure of methane-hydrogen flames, and validated through use of the correlation coefficient. Further experimental investigations of the low-swirl methane-hydrogen flames are performed by this computed tomography of chemiluminescence (CTC) system. The flame properties are determined to evaluate the effects of various fractions of blended hydrogen on the flame structure, including the flame center of mass, flame surface density and fractal dimension. Results showed that the flame initially has bowl-shape structure. The flame height gradually decreases and the flame cross-sectional area increases as the blended hydrogen fraction increases. Meanwhile, the bottom of the flame moves toward the nozzle outlet. Nevertheless, when the blended hydrogen fraction reaches 50%, the flame transforms into a cup-shaped flame and shows a double-layer flame structure. When the flame is bowl-shaped, the flame center of mass moves away from the nozzle center axis with increasing blended hydrogen fraction, but the fractal dimension and flame surface density are not sensitive to the blended hydrogen fraction. In contrast, when the flame is cup-shaped, the flame center of mass moves towards the nozzle center axis, and the flame surface density and fractal dimension increase significantly, indicating the increase of flame wrinkling. These results reveal that blending hydrogen leads to two distinct flame structures and characteristics in low-swirl flame, also demonstrate that the proposed 3D low-cost CTC system based on multiple UV cameras is well-suited for combustion diagnostics.

Effect of temporal increase in equivalence ratio on combustion instability of a lean-premixed swirling hydrogen flame

The effect of temporal increase in the equivalence ratio on the combustion instability of a lean-premixed low-swirl hydrogen jet flame in a low-swirl combustor (LSC) is investigated in detail using a high-fidelity Large-Eddy Simulation (LES). The equivalence ratio is linearly increased from 0.3 to 0.5 over a duration of 0.4 s. The results show that the pressure oscillation amplitude in the combustor increases significantly when the equivalence ratio at the combustor inlet (ERCI) exceeds 0.42, and the maximum pressure amplitude and the combustion instability mode exhibit trends consistent with those in a previous experiment and numerical simulation conducted with the same LSC setup at a fixed equivalence ratio of 0.39. Temporal variations in the equivalence ratio and consequently the temperature inside the combustor cause the drastic amplification of pressure oscillation (when the ERCI exceeds 0.42) whose amplitude is larger than that at the fixed equivalence ratio (= 0.39). Prior to the onset of this exceptionally strong combustion instability, a transient irregular oscillation phenomenon comprising instantaneous changes in the pressure oscillation frequency is observed. While the pressure oscillations in the combustor and in the injector channel are in phase after the onset of strong combustion instability, they are in opposite phases during the occurrence of the irregular oscillation phenomenon prior to the onset of strong combustion instability. This irregular oscillation phenomenon predicted by the LES may play a crucial role in the mechanism of transition from stable combustion to combustion instability.

Reacting Flow Prediction of the Low-Swirl Lifted Flame in an Aeronautical Combustor With Angular Air Supply

Abstract The development of lean-burn combustion systems is of paramount importance for reducing the pollutant emissions of future aero engine generations. By tilting the burners of an annular combustor in circumferential direction relative to the rotational axis of the engine, the potential of increased combustion stability is opened up due to an enhanced exhaust gas recirculation between adjacent flames. The innovative gas turbine combustor concept, called the short helical combustor (SHC), allows the main reaction zone to be operated at low equivalence ratios. To exploit the higher stability of the fuel-lean combustion, a low-swirl lifted flame is implemented in the staggered SHC burner arrangement. The objective is to reach ultralow NOx emissions by complete evaporation and extensive premixing of fuel and air upstream of the lean reaction zone. In this work, a modeling approach is developed to investigate the characteristics of the lifted flame in an enclosed single-burner configuration, using the gaseous fuel methane. It is demonstrated that by using the large eddy simulation method, the shape and liftoff height of the flame are adequately reproduced by means of the finite-rate chemistry approach. For the numerical prediction of the lean lifted flame in the SHC arrangement, the focus is on the interaction of adjacent burners. It is shown that the swirling jet flow is deflected toward the sidewall of the staggered combustor dome, which is attributed to the asymmetrical confinement. Since the stabilization mechanism of the low-swirl flame relies on outer recirculation zones, the upstream transport of hot combustion products back to the flame base is studied by the variation of the combustor confinement ratio. It turns out that increasing the combustor size amplifies the exhaust gas recirculation along the sidewall, and increases the temperature of recirculating burned gases. This study emphasizes the capability of the proposed lean-burn combustor concept for future aero engine applications.

Mechanisms Leading to Stabilization and Incomplete Combustion in Lean CH4/H2 Swirling Wall-Impinging Flames

Abstract In gas turbines, confined highly turbulent flames unavoidably propagate in the vicinity of a relatively cool combustor liner, affecting both the local flame structure and global operation of the combustion system. In our recent work, we demonstrated, using simultaneous [OH] × [CH2O] planar laser-induced fluorescence (PLIF) and stereo-particle image velocimetry (stereo-PIV), that lean CH4/H2 flames at a high Karlovitz number can present a highly broken structure near wall, highlighted by a diffuse CH2O cloud, which suggests local quenching and incomplete oxidation. Such high Karlovitz numbers were achieved using an inclined plate, which substantially extended the lean flammability of the low swirl flames. Yet, how a cooled wall acting as a heat sink played a conducive role in stabilizing high Ka flames remains unanswered. In addition, the origin of the CH2O cloud is also unclear. Hence, in this work, we look to better understand the stabilization mechanisms for lean and ultralean flames on the same configuration, and how they may change with a parametric variation of plate incident angle, plate-nozzle distance, and bulk velocity up to the critical values that lead to flame blow off. The results show that the impinging swirling flow creates a low speed region that helps hold the flame, while the wall prevents mixing with ambient cold air. The production of diffuse CH2O, which indicates the occurrence of local quenching, is associated with a mean strain rate K beyond the extinction strain rate (ESR) Ke. For CH4 flames, most of the reaction zones reside within |K|/Ke<1; for 70% H2 flames at ϕ=0.4, the reaction zones are highly broken and scattered in a large area, where |K|/Ke<8, the interspace of which is fully filled by CH2O. In other words, high H2 fraction flames appear to be more robust to persistent strain rate, thus extending their stability envelope. However, these flames can subsist as highly broken flames featuring strong incomplete combustion.

Modeling Edge Flame Propagation for Flame Particle Tracking in the Forced Ignition Process of Turbulent Nonpremixed Jet Flame

ABSTRACT To ensure safe and reliable operation, ignition performance in propulsion and power devices needs to be thoroughly assessed. Reduced order models (ROMs) for ignition have gained increasing attention, given that first-principles models are restricted by immense computational costs that preclude rapid assessment for the design purpose. Currently, flame self-propagation is generally neglected in the ROM descriptions of ignition process and the impact on the flame front dynamics as well as the ignition probability map is not clear for low swirl or jet flames which are of interest for industrial applications. In this study, an empirical model for flame self-propagation is proposed to augment the capability of Lagrangian flame particle tracking method, a ROM model to predict ignition probability based on nonreacting flow field. The model adopts the classic scaling law of turbulent flame speed together with fuel gradient correction and flame self-propagation to mimic the edge flame dynamics during the ignition process. The model performances in terms of flame front dynamics and ignition probability map are quantified in a turbulent nonpremixed methane-air jet flame. Results reveal that only with flame self-propagation, the flame particle tracking can well capture the transient evolution of the leading flame front as observed in experiments. It is found that the predicted ignition probability map agrees well with experimental data with high ignition probability around the vicinity of the stoichiometric region at the downstream axial distance of 6 to 40 times the jet diameter. This study illustrates the importance to account for flame self-propagation to correctly predict ignition probability in jet dominant nonpremixed flames.

Numerical investigation of wall effects on combustion noise from a lean-premixed hydrogen/air low-swirl flame

A hybrid computational fluid dynamics (CFD)/computational aero-acoustics (CAA) approach, in which large-eddy simulation (LES) and APE-RF (solution of the acoustic perturbation equations for reacting flows) are employed for the CFD and CAA, respectively, calling it the hybrid LES/APE-RF approach, is used to analyze the influence of a wall on the combustion noise from a lean-premixed gaseous hydrogen/air low-swirl turbulent jet flame. The wall boundary conditions pertaining to the APE-RF system are formulated to account for acoustic reflection from the wall. The results show that the sound pressure level (SPL) spectrum obtained from the LES/APE-RF is in good agreement with that measured in the experiment. In the LES/APE-RF, the SPL spectrum of combustion noise with the wall plate explicitly changes compared to that without the wall plate. Specifically, the presence of the wall plate tends to ease the peaks that appeared in the case without the wall plate and create a nearly constant SPL within a specific frequency band. The analysis of the heat release rate fluctuation reveals that these phenomena are caused by the absence of a single periodic oscillation of heat release rate. The presence of the wall plate creates an asymmetric flow around the flame and distorts the flame structure, thereby altering the flame fluctuation phenomena.

Detailed unsteady dynamics of flame-flow interactions during combustion instability and its transition scenario for lean-premixed low-swirl hydrogen turbulent flames

This experimental study elucidates the unsteady dynamics of flame-flow interactions during unique thermoacoustic instability (TI) and the transition mechanism from stable combustion to TI for lean-premixed hydrogen turbulent jet flames in a low-swirl combustor (LSC), where a swirler assembly consists of an unswirled central region (CR) and an annular swirler region (SR). Simultaneous 200-kHz pressure fluctuation p’ measurements and 10-kHz OH* chemiluminescence imaging, as well as 40-kHz stereoscopic particle image velocimetry (SPIV) and two-dimensional PIV measurements for steady-state and transient data acquisitions, respectively, were conducted. The SPIV was performed in multiple planes to explore three-dimensional velocity fields. During TI, periodic flashback was possibly caused by significant axial velocity oscillations, resulting in the local mixture velocity falling below the turbulent flame speed. The large-scale vortex ring generated by the velocity oscillations caused axisymmetric radial velocity Vr oscillations with switching signs during the TI period. Similar to a typical low-swirl flow, the positive Vr away from the combustor axis created diverging flow, whereas unlike the typical flowfield, the negative Vr toward the combustor axis generated converging flow while flattening the axial velocity distributions, which was the signature phenomenon for this TI. Using the transient data and dynamic mode decomposition, variations in delay times between the mixture injection and its convection to a region with positive local Rayleigh indices were investigated. During stable combustion, the mixture jet from the SR predominantly induced thermoacoustic coupling (TC). As the combustion transitioned into the TI, the mixture jet from the CR began to induce TC and, eventually, achieved predominance in inducing TC during fully evolved TI. The transition from the SR jet- into CR jet-dominant TI arising from the dynamic flame-flow interactions resulted from the inherent physical characteristics of hydrogen flames, thereby yielding the larger p’ amplitude compared to typical TIs.

Effect of fuel reactivity on flame properties of a low-swirl burner

The present paper examines the capability of low swirl flows in stabilizing low calorific value fuels. To such an aim, CO2 gas is used to adjust the reactivity of CH4/air mixtures. The results show that increases in CO2 percentage in the fresh mixture result in shrinking the burner stability map by moving the lean and rich blowout limits toward the stoichiometric condition. The effects of CO2 addition on rich blowout limits are more intense than the lean blowout limits. For instance, the equivalence ratio associated with the lean blowout increases by 0.23, when the discharge flow velocity from the burner rises from 1.5 to 5 m/s in the 40 % CO2 diluted case. Under a similar augmentation in the burner discharge velocity, the rich blowout limit reduces by more than 0.5 in the 40 % CO2 diluted case. Unlike the lean blowout, the rich blowout limit responds non-monotonically to the discharge flow velocity from the burner for both undiluted and diluted mixtures. For the 50 % CO2 diluted case, the low swirl burner operates efficiently at low discharge velocities (1.5 to 3.5 m/s), but it blows out at high discharge velocities (3.5 to 9 m/s). Furthermore, unsteady features of low swirl flame near blowout limits are studied in detail. Moreover, strong curvy structures appear at the flame boundary prior to the flame flashback. However, such structures diminish occasionally and specifically as the flame approaches the flashback. Some deterministic features are more obvious before the flame blowout than those before the flame flashback.

Investigation of combustion noise generated by an open lean-premixed H[formula omitted]/air low-swirl flame using the hybrid LES/APE-RF framework

An open lean-premixed hydrogen/air low-swirl (LPHALS) turbulent flame exhibiting a pronounced peak in its combustion noise spectra, is investigated numerically using a hybrid Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) framework. Under this framework, the reacting flow-field of the flame is computed via Large-Eddy Simulation (LES), while the direct combustion noise it produces is captured by solving the Acoustic Perturbation Equations for Reacting Flows (APE-RF). Flame configuration and simulation conditions correspond to those of an experimental study on an open lean-premixed H2/air flame stabilized using a Low-Swirl Burner (LSB). LES results are validated against experimental data. The CAA simulation is able to predict a pronounced sharp peak in the computed combustion noise spectra, similar to one of the two characteristic peaks observed in the measured combustion noise spectra. Frequency of this spectral peak predicted by the CAA simulation is 840 Hz, which is close to that of the higher frequency secondary spectral peak at 940 Hz measured in the experiment. Upon examining the hybrid LES/APE-RF results, the noise generation mechanism at 840 Hz is found to be the intense local heat release rate fluctuations, caused by strong interaction between the flame and the periodically generated vortical flow structures in the shear layers, downstream of the LSB exit. Additionally, analysis of the spectral content and directivity of the noise generated by different acoustic source terms is performed, in order to investigate their impact on the radiated acoustic field, and hence the characteristics of direct combustion noise produced by the open LPHALS flame.

막힘률에 따른 저선회 화염 동역학에 대한 실험적 연구

The influence of the blockage ratio on low-swirl flame dynamics was experimentally investigated in a lean-premixed combustor. Here we test five different swirl injectors, one with high swirl and four with low swirl, all with different blockage ratios (β). We analyze a large experimental dataset of self-excited instability measurements, and show that instability amplitudes for the case of β = 0.75 tend to be remarkably attenuated. This is attributed to the development of out-of-phase modulations between the jet-stabilized flame in the inner section and the swirl-stabilized flame in the outer section.

Experimental Investigation on Nonlinear Response of a Low-Swirl Flame to Acoustic Excitation With Large Amplitude

Abstract Thermoacoustic instability is a major issue in developing high-efficiency low emission gas turbine combustors. In order to predict the amplitude of limit cycle oscillation, an understanding of the amplitude-dependent response of the flame, i.e., the nonlinear response, to large acoustic excitation is needed. In the present study, the nonlinear response of a low-swirl CH4/air premixed flame to acoustic excitation is experimentally studied. Amplitude dependences of flame dynamic at 75 Hz and 195 Hz are discussed in detail over a wide range of excitation levels. Experimental results show the gain of flame describing function of the low-swirl flame has a peak value at 65 Hz and a local minimum at 105 Hz which is caused by the destructive (out of phase) and constructive (in-phase) of the axial and azimuthal velocity fluctuation. At low perturbation level, flame heat release fluctuation is in linear relationship with the normalized velocity driving level. Heat release fluctuation begins to saturate at a certain level which depends on the driving frequency. The low-swirl flame oscillates mainly in the axial direction at 75 Hz while it is in the radial direction at 195 Hz. The nonlinear flame heat release response is a result of combination effect of flame rollup process and harmonic responses.

난류생성기 판 형상에 의한 난류화염 특성 연구

The cross-shaped fractal grids which have RRBT(reduced ratio of bar thickness) and the hexagonal grid were used to generate various turbulent flow properties for low swirl flames. In the non-reacting flow field with the fixed bulk velocity at 10 m/s, decreasing RRBT induces the increasing turbulent intensity(ISUBU/SUB) with the delay in the location of lowest turbulent mean velocity(Ū) and highest turbulent fluctuation(ú) due to vortex shedding from the bar thickness. This phenomenon was well explained by a wake interaction theory using axial velocity profiles at the centerline. At various equivalence ratios for Hexa. and fractal grids, the characteristics of low swirl flames could be identified to a thin reaction zone regime in the Borghi – Peter regime diagram. Decreasing RRBT increases ISUBU/SUB in front of flame brush with increasing turbulent local displacement speeds (SSUBT,LD/SUB) via a linear correlation. The turbulent Reynolds number, ReSUBT/SUB composed of the ratio of length scale and velocity components induced the proportional trends in non linear relationship showing that the influence of the length scale ratio. The empirical correlation study of SSUBT,LD/SUB as a function of ReSUBT/SUB and ú/SSUP0/SUPSUBL/SUB was investigated to compare the influence of velocity and turbulent length scales ratio. The result of linear correlation equation with SSUBT,LD/SUB indicates the influence of velocity factors greater than ReSUBT/SUB through the exponential factor.

Nonlinear response of a premixed low-swirl flame to acoustic excitation with large amplitude

This paper describes an experimental investigation of the nonlinear response of a premixed methane/air low-swirl flame to imposed acoustic excitation with perturbation amplitude up to 95% of the bulk velocity. Amplitude dependence of flame dynamic at a low frequency,75 Hz, and a high frequency, 195 Hz, over a wide range of excitation level is analyzed in detail. Proper orthogonal decomposition is used to identify the dominant structures in the low-swirl flame. Experimental results show the gain of FDF of the low-swirl flame has a peak value at 65 Hz and a local minimum at 105 Hz which is caused by the destructive (out of phase) and constructive (in phase) interference of the axial and azimuthal velocity fluctuations. The low-swirl flame shows complex nonlinear amplitude dependence which depends on the excitation frequency. Flame harmonic response has a close relation with non-linearity behavior. Vortices are formed and convected downstream along the flame approximately at bulk flow velocity, inducing flame rollup and shear layer instabilities. Fluctuation of heat release rate in the flame tail region and shear layer due to the combined impact of flame rollup and shear layer instabilities govern the flame nonlinear response. Further POD analysis shows the most energetic modes (mode 1 and mode 2) feature symmetric wave-like structures. As the perturbation level increases, the higher anti-symmetric and helical modes are inhibited.

Effects of Flame Behaviors on Combustion Noise from Lean-Premixed Hydrogen Low-Swirl Flames

This experimental study explored the influence of global/local flame behaviors on direct combustion noise produced by a lean-premixed gaseous low-swirl turbulent jet flame, with a focus on the mechanism resulting in characteristic peaks in combustion noise spectra. Ten kilohertz chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) imaging were used to study the spatiotemporal evolution of heat release and flame structure fluctuations, respectively, whereas 2-D particle image velocimetry (2-D PIV) measurements at 4 Hz were applied to study the mean-based relation between the velocity/vorticity fields and flame structures. Pressure and global heat release fluctuation measurements carried out alongside these optical diagnostics revealed pronounced double peaks in both combustion noise and global heat release fluctuation spectra at global equivalence ratios of . Spectral proper orthogonal decomposition of the chemiluminescence and OH-PLIF images revealed that the first peak in the noise spectra to be caused by flame oscillations over nearly the entire flame region, whereas the secondary peak was attributed to periodically generated vortical flame structures near the downstream side of the flame boundary. The 2-D PIV results suggest that vortical flame structures are likely generated by the interaction between the flame and the inner/outer shear layers.

OH* and CH* chemiluminescence characteristics in low swirl methane-air flames

Chemiluminescence information is of great significance for characterization of flame structure and combustion characteristics. An atmospheric low swirl burner was developed to investigate the chemiluminescence characteristics of OH* and CH* in low swirl flames, with the equivalence ratio varying from 0.8 to 1.2 and the swirl number from 0.2 to 0.6. The chemiluminescence images were captured via ICCD cameras coupled with narrow-bandpass filters, and an Abel inversion method was introduced to transform the line-of-sight-integrated image into two-dimensional radial distributions. The results show that the distribution of CH* is smaller than that of OH* and concentrated more upstream of the flame near the burner. The equivalence ratio has a relatively more direct influence on chemical reactions, while the swirl number has a more evident effect on the flame structure. As the equivalence ratio increases, the peak value of OH* and CH* increases and the peak position moves downstream of the flame, suggesting that the chemical reactions become more intense. In contrast, the height and width of chemiluminescence distribution increase linearly with increasing swirl number. Moreover, it is found that the equivalence ratio and swirl number can be feasibly estimated based on chemiluminescence measurement results, using the correlation between them derived from this study.

Experimental study on the excitation of thermoacoustic instability of hydrogen-methane/air premixed flames under atmospheric and elevated pressure conditions

The current work examines the excitation of thermoacoustic instability of lean premixed hydrogen-methane/air low swirl flames under both atmospheric and elevated pressure conditions (up to 0.3 MPa). Under a given pressure condition, The tests were conducted at different bulk velocities (U), hydrogen proportions (ηH), and equivalence ratios (Ф). Results show that thermoacoustic instability can be excited by increasing one of these variables while keeping others the same. It was found that pressure elevation has a minor effect on the oscillation frequency. Moreover, it was demonstrated that the current instability is induced by large coherent structures. The effect of pressure elevation on the excitation of thermoacoustic instability is found to be Φ dependent. As indicators of the flame response to impinging vortices, the curvature and local flame surface area features were calculated with images captured with the planar laser-induced fluorescence of the OH radical (OH-PLIF) method. Results demonstrate a great similarity between the flame front evolution and the instability trend, implying that the effect of the chamber pressure on the instability trend can be indicated by the change in the flame front curvature and local flame surface area.